An electroluminescent (EL) device is a device which, while making use of the phenomenon of electroluminescent, emits light when the device is suitably connected to a power supply. If the light emission originates in an organic material, said device is referred to as an organic electroluminescent device. An organic EL device can be used, inter alia, as a thin light source having a large luminous surface area, such as a backlight for a liquid crystal display or a watch. An organic EL device can also be used as a display if the EL device comprises a number of EL elements, which may or may not be independently addressable.
The use of organic layers as an EL layer in an EL element is known. A class of said known organic layers generally comprise a conjugated, luminescent compound. Said compound may be a low-molecular dye, such as a coumarin, or a high-molecular compound, such as a poly(phenylenevinylene). The EL element also comprises two electrodes, which are in contact with the organic layer. By applying a suitable voltage, the negative electrode, i.e. the cathode, will inject electrons and the positive electrode, i.e. the anode, will inject holes. The recombination of these electrons and holes may generate light. If the EL element is in the form of a stack of layers, at least one of the electrodes should be transparent to the light to be emitted. A known transparent electrode material for the anode is, for example, indium tin oxide (ITO). Known cathode materials are, inter alia, Al, Ba, Yb, Mg:Ag, Li:Al or Ca. Known anode materials are, in addition to ITO, for example, gold and platinum. If necessary, the EL element may comprise additional organic layers, for example, of an oxadiazole or a tertiary amine, which serve to improve the charge transport or the charge injection.
An EL device of the type mentioned in the opening paragraph is disclosed in EP-A-0 893 939, which describes an electroluminescent display device comprising an organic electroluminescent element formed on a substrate and an organic compound layer group sandwiched between cathodes and anodes. The organic compound layer group has laminated electroluminescent functional layers formed of at least one kind of organic compound. The organic electroluminescent display device further includes an airtight case that encloses the organic electroluminescent element with a space formed between the airtight case itself and the organic electroluminescent element and isolates the organic electroluminescent element from outside air.
As disclosed in EP-A-0 893 939, there may be a small risk of dust adhesion on the organic EL element. Since the organic EL functional layer has a very small thickness of a submicron order, a short circuit due to the dust is liable to occur between the transparent electrode (anode) and the back electrode (cathode). The conventional countermeasure for preventing short circuits are washing and smoothing of the surface of the substrate. However, as it is impossible to perfectly clean the substrate surface, there still remains possibility of occurrence of short circuits.
Also the influence of moisture, oxygen or the like, when permeated into the organic layer via a defect in, for example, the cathode layer, may cause deterioration of the organic electroluminescent functional layer.
In order to reduce the risk of a short circuit, EP-A-0 893 939 employs a filler gas filling the space within the airtight case. Said filler gas includes at least one kind of combustion-supporting gas. The combustion-supporting gas oxidizes the cathode material of a portion surrounding a short circuit to form an insulator, and thereby suppresses the occurrence of a short circuit. A combustion-supporting gas is a gas-oxidizing agent that does not burn itself, but helps combustion of other materials. Known combustion-supporting gasses are, e.g. oxygen, dinitrogen oxide, ozone, chlorine, nitrogen monoxide, fluorine, nitrogen trifluoride, oxygen difluoride, etc.
The necessary encapsultation of a filler gas implies strong restrictions on future architectures of organic electroluminescent devices.
The invention aims to provide a reliable organic electroluminescent device as described in the first paragraph where the risk for failure before the end of its lifetime, is strongly reduced.